The statements in this section merely provide background information related to the present disclosure. Accordingly, such statements are not intended to constitute an admission of prior art.
Known spark-ignition (SI) engines introduce an air/fuel mixture into each cylinder which is compressed in a compression stroke and ignited by a spark plug. Known compression ignition engines inject pressurized fuel into a combustion cylinder near top dead center (TDC) of the compression stroke which ignites upon injection. Combustion for both gasoline engines and diesel engines involves premixed or diffusion flames controlled by fluid mechanics.
SI engines can operate in a variety of different combustion modes, including a homogeneous SI combustion mode and a stratified-charge SI combustion mode. SI engines can be configured to operate in a homogeneous-charge compression-ignition (HCCI) combustion mode, also referred to as controlled auto-ignition combustion, under predetermined speed/load operating conditions. The HCCI combustion mode includes a distributed, flameless, auto-ignition combustion process that is controlled by oxidation chemistry. An engine operating in the HCCI combustion mode has a cylinder charge that is preferably homogeneous in composition, temperature, and residual exhaust gases at intake valve closing time. HCCI combustion is a distributed kinetically-controlled combustion process with the engine operating at a dilute air/fuel mixture, i.e., lean of a stoichiometric air/fuel point, with relatively low peak combustion temperatures, resulting in low NOx emissions. The homogeneous air/fuel mixture minimizes occurrences of rich zones that form smoke and particulate emissions.
Operation in HCCI combustion mode, wherein combustion occurs based upon compression of the charge to a point of substantially uniform combustion throughout the combustion chamber, is highly dependent upon conditions within the combustion chamber. If insufficient heat and/or pressure is present within the combustion chamber, the charge may fail to ignite or may misfire, resulting in unstable combustion or a drop in efficiency of combustion. If excessive heat and/or pressure is present within the combustion chamber, the charge may combust before the intended start of combustion, resulting in disorderly combustion or ringing in the cylinder.
An engine operating in HCCI combustion mode can be operated in a spark assist mode, wherein air and fuel are introduced to the combustion chamber in proportion to operation in the HCCI combustion mode, while a spark from a spark plug is utilized in advance of anticipated auto-combustion to ensure proper combustion.